The present invention relates to the production of polyamides by the anionic polymerization of lactams.
Polyamides, particularly polylactams which are a class of polyamides prepared by polymerizing lactam monomers, exhibit a wide range of properties suitable for a variety of applications. Polycaprolactam, such as that available as NYLON 6, is perhaps the most widely used polyamide because it exhibits excellent mechanical and physical properties at a moderate cost.
Polycaprolactam can be polymerized under substantially anhydrous conditions by anionic catalytic polymerization. That is, the process is initiated with a strong base. Such suitable catalysts for the polymerization are lactam anions which can be obtained from lactam-metal compounds having a metal atom, typically an alkali or alkaline earth metal, bound to their nitrogen atom. For example, sodium caprolactamate, potassium caprolactamate, and Grignard compounds such as caprolactam magnesium bromide can be utilized.
The anionic polymerization process can be appreciably accelerated by the addition of an initiator or promoter to the polymerization reactants. For example G.B. Pat. No. 1,099,265; G.B. Pat. No. 1,067,153; and U.S. Pat. No. 3,308,099 describe the addition of an initiator to accelerate polymerization. The promoters or initiators which have been suggested by the art include, for example, compounds containing a tertiary nitrogen atom to which are bound two or more carbonyl, thio carbonyl, sulfonyl or nitroso groups; compounds which react with a lactam monomer to form one of these aforelisted compounds, for example isocyanates, acid chlorides, or acid anhydrides, and compounds such as bislactams, for example, terephthaloyl bis caprolactam. Polymerization is generally carried out by using a single catalyst in conjunction with a single type of initiator.
Uses of polycaprolactam include the production of fibers, fabrics, as well as molded and laminated articles. For some applications a combination of properties is desirable, for example, impact resistance, stiffness, heat sag resistance, rapid cure and reduced water absorption; hence polymers of polycaprolactam plasticized with various polymeric polyols are generally prepared to suit particular demands. The preparation of these plasticized polymers, however, is not without attendant difficulties. Often, certain desirable properties are diminished, such as rapid cure with a given level of catalyst and initiator. Moreover, other difficulties can arise, for example, the presence of high levels of unreacted lactam monomer can give rise to a malodorous finished product.
Over the years efforts have been directed toward the goals of making the polymerization of lactams a more efficient process and at the same time achieving a multiplicity of desirable properties in the finished product. A simple, economical, and expeditious manner of achieving these goals is needed.